1. Field of the Invention
The present invention relates to a non-luminous flat display device, and more particularly, to a backlight assembly of a liquid crystal display with an improved optical sheet.
2. Description of the Related Art
Recently, a variety of flat panel displays have been developed to reduce weight and volume acting as drawbacks of cathode ray tubes (CRTs). Such flat panel displays include a liquid crystal display (LCD), a field emission display, a plasma display panel (PDP), an electro-luminescence (EL) and the like. Research for enhancing the display quality and increasing the screen size of the flat panel displays is being actively performed.
Among such flat panel displays, LCDs have several advantages including slimness, light weight, low power consumption characteristics. In particular, the LCD displays information using electrical and optical properties of liquid crystal molecules. The LCD itself is a non-luminous device that displays images using a light source such as a lamp. In other words, unlike in the CRT, the LCD essentially needs a separate unit for irradiating light, i.e., a backlight assembly because the liquid crystal molecules injected between a thin film transistor (TFT) substrate and a color filter substrate does not emit light on their own.
In general, a backlight assembly includes a mold frame having a receiving space, a reflector mounted on the lowest surface of the receiving space for reflecting light toward liquid crystal display panel, a light guide plate disposed on the reflector for guiding light, a lamp unit disposed between the light guide plate and a sidewall of the receiving space for emitting light, optical sheets stacked on the light guide plate for diffusing and focusing light, and a top chassis disposed on the mold frame and enclosing the liquid crystal display from a predetermined portion of the edge of the liquid crystal display panel to a side portion of the mold frame.
The optical sheets include a diffusion sheet for diffusing light, a prism sheet disposed on the diffusion sheet for focusing the diffused light and transferring the focused light to the liquid crystal display panel, and a protection sheet for protecting the diffusion sheet and the prism sheet.
FIG. 1 is a sectional view of a related art LCD.
Referring to FIG. 1, the related art LCD includes a backlight assembly 50 for generating light, and a display unit 40, which receives the light from the backlight assembly 50 and displays images. The backlight assembly 50 includes a lamp unit 51 for generating light and a light guide unit for guiding the light generated by the lamp unit 51 toward a liquid crystal display panel 10.
The display unit 40 includes the liquid crystal display panel 10 and an upper polarizer 30 and a lower polarizer 20 respectively disposed on an upper surface and a lower surface of the liquid crystal display panel 10. The liquid crystal display panel 10 includes a TFT substrate 11 and a color filter substrate 12 each having an electrode formed thereon.
Specifically, the lamp unit 51 includes a lamp 51a for generating light and a lamp reflector 51b enclosing the lamp 51a. The light generated by the lamp 51a is incident into a light guide plate 52 to be described later. The lamp reflector 51b reflects the light generated by the lamp 51a toward the light guide plate 52, thereby increasing amount of incident light toward the light guide plate 52.
The light guide unit includes a reflection plate 54, the light guide plate 52, and optical sheets 53. The light guide plate 52 is disposed at one end of the lamp unit 51 to guide the light emitted from the lamp unit 51 such that the light emitted from the lamp unit 51 is incident into the LCD panel 10.
The reflection plate 54 is disposed beneath the light guide plate 52 and reflects light leaked from the light guide plate 52 toward the light guide plate 52 again.
The plurality of optical sheets 53 are disposed on the light guide plate 52 to enhance efficiency of the light passing through the light guide plate 52. Specifically, the optical sheets 53 include a diffusion sheet 53a, a prism sheet 53b and a protection sheet 53c, and are orderly stacked on the light guide plate 52.
The diffusion sheet 53a scatters the light that is incident from the light guide plate 52, thereby making the brightness distribution of the light uniform. The prism sheet 53b includes a plurality of prisms formed repeatedly on an upper surface thereof and focuses the light diffused by the diffusion sheet 53a in a direction perpendicular to a plane of the LCD panel 10. Accordingly, the light that has passed through the prism sheet 53b mostly advances in a direction perpendicular to the plane of the LCD panel 10, thereby obtaining a uniform distribution.
The protection sheet 53c provided on the prism sheet 53b functions not only to protect the upper surface of the prism sheet 53b, but also to diffuse light so as to make the distribution of the light that is incident from the prism sheet 53b uniform.
FIGS. 2A and 2B are a sectional view and a perspective view of the prism sheet illustrated in FIG. 1 respectively.
Referring to FIGS. 2A and 2B, the related art prism sheet 200 includes a body part 210, through which the light diffused by the light guide plate and the diffusion sheet is initially introduced, and a protrusion part 220 shaped in a isosceles triangle prism, for maintaining path of the diffused light constantly. The protrusion part 220 has a plurality of isosceles triangle prisms linearly arranged in a stripe form.
The protrusion part 220 shaped in the isosceles triangle prism has a pitch ranging from a few tens μm to a few hundreds μm, and an apex angle α ranging from 60 degrees to 130 degrees. When the apex angle α is an acute angle, the brightness increases but the viewing angle decreases.
Also, when the protrusion part 220 of the isosceles triangle prism is installed directing toward the front side of the LCD panel, the prism sheet 200 focuses the diffused light introduced into the body part 210 by refracting the diffused light toward the front side, but the light that is introduced into inclined surfaces of the protrusion part 220 does not contribute to the enhancement of the front brightness and is lost due to the inner full reflection.
To overcome this drawback, the prism sheet may be arranged such that the protrusion part 220 is directed toward the light guide plate. However, such an arrangement enhances the front brightness, but decreases the viewing angle, which is counter to use in the flat panel displays requiring a wide viewing angle, such as a home television.